Traffic Shapers - High Priority VoIP Phones - Other Lower Priority Tasks

We have some locations that have VoIP phones. We want to guarantee that these phones have the required bandwidth that they need when in use. We don't want though to waste bandwidth on the phones when none of the phones or only some of the phones are in use. We want bandwidth not needed by the phones to be used for other users or for backup tasks. If the phones use 100 Kbps each how is this best managed?

I was going to set a Per-IP Shaper but I see that these traffic shapers only regulate the maximum bandwidth, not the minimum bandwidth per IP (phone).

Setting a Shared Shaper and setting a Guaranteed Bandwidth based on the bandwidth per phone X the number of phones would appear to guarantee enough bandwidth but what happens if not all of the phones are in use? Would the unused bandwidth be wasted?

What would happen if a Shared Shaper was set with a High Traffic Priority with no Max Bandwidth or Guaranteed Bandwidth set? Would using this Shared Shaper as the Shared Shaper and Reverse Shaper first on the list of Traffic Shaping Policies give the phones in use all that they need but leave all leftover traffic for other uses?

FortiOS 6.0.9